Right, first for some back ground!

When I first got into amateur telescope making I noticed that the blanks for the size and type of telescope I really wanted to build were, at least for me, being based in Australia, priced way outside my budget. So I started to look into the fesablity of casting my own blanks. The more I looked into it the more I realised that, although not a simple undertaking, with some hard work and the right equipment it would be relatively easy to do.

So I started to track down the equipment that I would need to make this "dream" come true. the first thing that I noticed was that the state of the art for precise temperature control for the serious amateur kiln user was really quite primative, and I would REALLY have to babysit the kiln during a casting session. Well this was 10 years ago, and now microprocessor controls for kilns have become relatively common and quite reasonably priced, so now I have finally embarked on the journey to the home cast telescope mirror blank!

One thing I did notice during my years of research was that getting the Pryrex/borosilicate glass required could prove a problem, so when and offer came my way for some free scrap pryrex I decided to jump, and so I secured about 110lbs of glass (Thank you Russ!), so once I took delivery of this glass, I really started to plan what I would need to make this happen, after several design iterations I settled on a kiln design that would allow me to cast blanks upto 13" in diameter, but while I was cutting the fire bricks for the walls my Dad made a suggestion to use the small wedge shaped scraps to expand the kiln's size. I said that this would not be the best way to do it, and so after a day of thought, I added five full bricks evenly spaced around the circumferance of the wall.

The addition of those five bricks has made the possiblity of casting blanks upto 16.5" in diameter a reality. Still not as big as I want to eventully go, but I'm getting there!

So now I have not a circular kiln, as I wanted, but a pentacular one instead. This is not a problem from a structural point of the more important thermal effiency side of things, but what it does give me is room for my hands when loading a mould of the maximum size, before with the circular kiln I had to not only leave room for the mould's walls, but also for my fingers, not I can make the mould that little bit bigger and I still have mould to wall clearance, and at five points around the perifery these is plenty of room for my fingers. Quite a bonous that!

Here are a couple of pics to give you an idea of the size of this project.



This is what the wall will look like, I haven't really started work on the floor or roof yet (they are of identical design and construction). You can see the slight pentacular shape to the cavity of the kiln.



Just to give you an idea of scale, that's a standard disposable lighter, the average internal "diameter", for want of a better term, is 18.5" now. However when the bricks are mortared together it will be close to 19" in diameter.

Well a few days have now passed and the walls have been mortared up, here is a picture of what the kiln now looks like.



I shrouded the soft fire brick walls with stainless steel sheeting to protect them from accidental knocks and to reduce the amount of dust that they will shed as it is quite bad for your health! BTW the new internal diameter is 19.125" on average, as the shapes isn't round.

So now all that remains to be done on the kiln body propper is to install the elements and the thermocouple probe, and mount the housing that will keep fingers away from the exposed terminals for those. So as I was still waiting for the TC probes I ordered to arrive I installed the elements.



Here is a detail shot of the transition area....

Now all that is really let to be done is make up the lid and install it's elements, make the hinge for it and install it. I desided to mortar up the lid today as I had the time to do it. Here are a couple of pictures of the newly made lid, I can't move it for a couple of days as the mortar has to cure.



The lid as made up.



A detail shot of the hinge plate, the hinge will be mounted on this plate, which is 2" by .25" steel and reaches around more than 50% of the lid's circumferance to spread the tortion forces when the lid is moved.



I used 3/4" "Band-it tm." clamps to hold everything together, I couldn't think of a better method that had the strength and the ability to spread the claping load as evenly. As you can see the forces are quite high, I just hope that they won't be too high!

Here is a picture of my proposed lid element layout, it's hard to ballance even spacing with a continueous loop layout!



Well the next update will be after I install the heating elements in the lid and do the hinge mechanism.

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